1. Field of the Invention
This invention relates to a hydraulic controller for automatic transmission use to control gear changing in a gearbox of an automatic transmission by hydraulic pressure.
2. Description of Related Art
An automatic transmission widely utilized in vehicles and the like performs gear changing control by switching, with a hydraulic valve, hydraulic pressure applied to several friction engagement devices to smoothly convey rotational driving force in accordance with load. Gear changing control is performed by manual operation by a rider to select either forward, neutral, or reverse by a select lever, or by automatic transmission to determine an engaged state of a friction engagement device so that a suitable gear ratio is obtained by an engine controlling computer from throttle opening or the like of the engine.
A torque converter to convey motive power of an engine to a gearbox with fluid as a medium in an input side of such a hydraulic controller for automatic transmission use according to the prior art is known. Because this torque converter employs fluid as a medium, vibrational shock can be absorbed, and there exists a function as a startoff device which permits suitable slippage and prevents engine stalls. Because the torque converter is inherently a slippage element, loss of motive power is unavoidable. In this regard, direct coupling of an input shaft to an output shaft (termed "lockup") is performed to improve the fuel consumption performance of the vehicle. This direct coupling of the input shaft to the output shaft (lockup) is not performed in the state of a first speed which is a low speed range. switching lockup on and off is generally performed by a solenoid to switch lockup on and off.
As a method for implementing this hydraulic controller in a simple manner, a structure jointly using a solenoid for lockup control use and a solenoid for use of friction engagement elements such as a brake, a clutch, and the like is disclosed in Japanese Patent Application Laid-Open Nos. Hei 6-26568 and 5-10430.
The system disclosed in the foregoing Japanese Patent Application Laid-Open No. 6-26568 jointly uses a duty solenoid to control a slip ratio of slip control of a lockup device in an engagement control switching valve of a low reverse brake. That is to say, a solenoid for lockup control use substitutes for engagement control of friction engagement elements operating outside a lockup region.
The system disclosed in Japanese Patent Application Laid-Open No. Hei 5-10430 causes the hydraulic systems and valve systems for two friction engagement elements which are not simultaneously supplied with hydraulic pressure, that is, which do not operate simultaneously, to be commonalized.
However, problem points such as the following exist in a hydraulic circuit for automatic transmission use disclosed in the foregoing publications commonalizing the above-described solenoid for lockup control use and solenoid for friction engagement element control use:
because the duty solenoid valve for controlling the slip ratio of lockup slip control becomes the pilot valve of the solenoid for lockup control use and the solenoid for friction engagement element control use, the number of parts is large and the hydraulic circuit is complex; PA1 because the switching device of the communicated destination of the foregoing duty solenoid valve is an on-off solenoid, the number of parts increases and the hydraulic circuit becomes complex, and current consumption increases, due to employment of a special solenoid; and PA1 when the lockup shift valve sticks or the on-off solenoid is locked, output pressure of the duty solenoid valve acts upon the lockup controller via a differential pressure control valve (i.e., the lockup control valve 50 disclosed in Japanese Patent Application Laid-Open No. Hei 6-26568), and so vehicle travel may be impeded.